Servo motor driven encoder error evaluation system

ABSTRACT

Disclosed is a system for evaluating errors in high precision encoders. A phase-lock servo system provides accurate and constant shaft speed control of an encoder under test and integration capability is provided to average either angle or time readings obtained during a plurality of revolutions of the encoder and thereby reduce the random errors originating in the testing apparatus. Also provided is a speed perturbation generator for checking the system&#39;&#39;s operability.

United States Patent [191 [111 3,794,899 Breslow Feb. 26, 1974 [54]SERVO MOTOR DRIVEN ENCODER ERROR 3,654,479 4/1972 Catherin 318/314 X[75] Inventor: Donald H. Breslow, Framingham m ry ner- E. Lynch Center,Mas Attorney, Agent, or Firm-Homer 0. Blair; Robert L.

- N th' ;W'll' C. R 'h [73] Assignee: Itek Corporation, Lexington, Mass.a 1 mm 0C 22 Filed: June 19, 1972 [57] ABSTRACT [21] Appl. N0.: 263,886

Disclosed is a system for evaluating errors in high precision encoders.A phase-lock servo system provides 1 [52] Cl 318/602 3 422 13 accurateand constant shaft speed control of an en- [51 I In G65) l9/28 coderunder test and integration capability is provided [58] i q l 8 328 toaverage either angle or time readings obtained dur- 3l8/359 333 b 6 iing a plurality of revolutions of the encoder and thereby reduce therandom errors originating in the References Cited testing apparatus.Also provided is a speed perturba- Y H tion generator for checking thesystems operahility.

UNl'l ED STATES PA'I EN l S 3,465,223 9/1969 Mears 318/329 X 22 Claims,3 Drawing Figures 32 PERTURBATION 3/ 27 22 27 GENERATOR r 34 REFERENCEENCODER Z8 INPUT F GENERATOR MOTOR 3 3a 29 23 g 35 CORRELATOR t A 23 3750 I, 50 il,

EVALU ATOR EVALUATOR EVALUATOR 26 24/ CIRCUIT CIRCUIT 5 CIRCUITEVALUATION SYSTEM PATENIEI) FEB 2 8 I974 A ENCODER U PERTURBATION -3GENERATOR MOTOR VALUATOR CIRCUIT EVALUATOR CIRCUIT EVALUATOR 24/ CIRCUITco RR ELATOR F/G. l.

V POSITION RECORDER & R E 0 m .6 I m v A 8 7 5 M w w w R m E A m R A A xU D- a o M 5 c m 3M 4 v T Cl 9 .mw m Lc K 4 4 f C 6 w 5 C L M S. Y R l rIL INTERVAL COMPARATOR CIRCUIT 69/ LOGIC 1 SERVO MOTOR DRIVEN ENCODERERROR EVALUATION SYSTEM BACKGROUND OF THE INVENTION This inventionrelates generally to encoders and more particularly to a system fortesting and evaluating errors in high precision encoders.

Optical encoders are widely used for such functions as analog to digitalconversion of radar shaft angles for computer inputs. As technologyadvances, the demands made of encoders become more rigorous, and

an ever increasing number of measurement points are required on eachencoder disc. Furthermore, an increasing accuracy is required for thedistribution of these points. Originally, encoders were tested by alimited point to point comparison with a standard polygon andautocollimator. However, with current demands for precisionmeasurements, limited point to point comparisons fail to yieldmeaningful results.

Presently, most encoders are tested by either autocollimators andpolygons or else by measuring the time required to traverse a specificangle as indicated by the calibrations on the encoder while rotating ata nominally constant speed. A tachometer system is usually used toprovide speed regulation. However, some encoders today require that theerror beless than 0.01 percent, and as a general rule the equipment usedfor calibration and checking should allow a safety factor'of three, orthe accuracy of the errorevaluation system should be in the order of0.0033 percent. Even the best dc tachometers are subject to dc drift andtorque fluctuations and are not capable of reducing shaft speed error toless than approximately 0.1 percent. Thus, the desired measurementaccuracy requires either better speed regulation, or a system that doesnot depend on precise speed control. However, systems of the latter typeinclude those employing reflective polygons, but such systems arenot-practical for high data rates since they require manual operation.

The object of this invention, therefore, is to provide segment. Sincethe average shaft speed in known, the

error, or difference betweenthe angle as'indicated by the calibrationson the encoder and the actual angle as indicated by the measured time,can easily be deter mined, and improved by multiple measurementaveraging.

One more feature is the utilization of a high inertia servo system.Speed regulation of a phase-lock servo system is provided by samplingthe outout, but the output of this servo will contain minorperturbations caused by the error in the calibrations of the encoder. Ahigh inertia system will not follow individual encoder errors in theoutput signal but will respond only to a longer average encoder outputand the error of the average output of any encoder approaches zero. Theservo system therefore will not follow and thereby hide individualerrors in the encoder.

Another feature of the invention is a correlator that adds to thecredibility of the error evaluation system by demonstrating that theservo system is not following individual encoder errors. The referenceinput signal is correlated with the transducer outputsignal and if theservo system were responding to the individual encoder Therefore, theoutput of the correlator is a constant for evaluating encoder errors anautomatic system that provides high speed operation. on a large numberof data points and exhibits a system error level in the order of lessthan 0.0033 percent.

SUMMARY OF THE INVENTION This invention is characterized by a phase-lockservo system that is responsive to a reference input signal and rotatesan encoder under test at nominally constant speed. The encoder undertest provides an output signal which is used for both feedback controland evaluation. The advantage of a phase-lock servo is that the longterm average accuracy of the speed control approaches the accuracy ofthe frequency control of the input signal although transientperturbations or molevel with white noise superimposed thereon, eachtransient perturbation giving rise to the white noise resulting from anerror on the encoder that is not followed by the servo system. Thus, thecorrelator shows that the servo system does not compensate for encodererror. I

Still another feature of'the invention is a perturbation generator thatfurther adds to the credibility of the system. The perturbationgenerator operates within the bandwidth of the servo and applies a knownerror signal to the reference input. With .the addition of theperturbation signal, the correlation of the reference input signal andthe encoder output signal yields an output with three components.Present are a constant level and a white noise component as previouslydescribed,

and superimposed thereon is a varying level proportional to theperturbation signal, thereby showing that the servo is following theperturbation signal.

Yet another feature of the invention is the utilization of a highlyaccurate crystal oscillator as a reference input frequency generator.The phase-lock servo system follows the input frequency, and precisioncrystal oscillators are made with accuracies of one part in 10",therefore the speed control of the phase-lock servo system approachesone part in 10 Another feature of the invention is an integrator thataverages the times measured during a plurality of traversals of theangular test segment, thereby further increasing the accuracy of thesystem. Increased accuracy results because minor torque variations maytemporarily perturb the shaft speed and cause error in an individualtraversal, but because the long term average error of shaft speedapproaches one part in 10 the average error caused by theseperturbations approaches one part in 10".

Another feature of the invention is the provision of a clock thatproduces a digital timing signal so as to permit a digital readout oftest results, and the utilization of digital techniques in errorevaluation.

' lar test segment. The accuracy of the oscillator can approach one partin thereby providing a very stable and accurate counting signal toreduce error in the evaluation system. Furthermore, a crystal oscillatorclock may be operated at high frequencies to produce several thousandpulses during traversal of the smallest angular test segment ofinterest. Quantization error is thereby eliminated as a practicalconsideration.

Another feature of the invention is the provision of a comparator in thesystem to automatically evaluate error by comparing the measure variableto a predetermined theoretical value. This technique reduces themagnitude of the numbers used in data reduction.

A feature of another preferred embodiment of the invention is theinclusion of a plurality of pairs of angular position sensors, each pairdefining and detecting an independent angular test segment and each pairassociated with a clock. Angular test segments may then be chosen fromall portions of the encoder and each angular test segment will bemeasured by a separate clock during each revolution of the encoder.Therefore many test segments may be measured simultaneously,substantially reducing the time required to test an encoder. This isadvantageous because long term average error of any encoder approacheszero so meaningful results can only be obtained from relatively small(approximately 2, for example) test segments and in order to test widelydispersed sections of the encoder many segments are necessary.

Another preferred embodiment of the invention combines one angularposition sensor, one interval timer and one angular position recorder.During operation of this embodiment the timer is automatically started,and at the expiration of a precise time interval the angular position asindicated by the output of the encoder is recorded. This recordedposition is then compared to the theoretical angular position ascomputed from shaft speed and the length of the time interval. In thisembodiment shaft position is the measured variable to determine error,as time is the variable measured in the previously described embodiment.Either embodiment'may be used depending upon which variable is moreconveniently measured and evaluated in the system of data reductionbeing used.

DESCRIPTION OF THE DRAWINGS These and other features and objects of theinvention will become more apparent upon a perusal of the followingdescription taken in conjunction with the accompanying drawings wherein:

F IG. 1 is a block diagram of a preferred embodiment of the invention; I

FIG. 2 is a block diagram of the detectors, clock and other datareduction components utilized in the embodiment shown in FIG. 1; and

FIG. 3 is a block diagram of the sensors, timer and data reductioncomponents utilized in an alternate embodiment of the invention.

DESCRlPTlON OF THE PREFERRED EMBODlMENT Referring first to FIG. 1 thereis shown a block diagram of a preferred embodiment of an encoder errorevaluation system 21 including a phase-lock servo systern 22, an outputof which is connected by a line 23 to a plurality of evaluation circuits24, 25....26. The evaluation circuit 24 is described in detail below. Inpractice, there need only be-one evaluation circuit 24, but more rapidand accurate evaluation is possible with a plurality of additionalevaluation circuits 25....26.

The servo 22 is responsive to a reference input signal that is carriedby a line 27 from a reference input generator 28. The reference inputgenerator 28 is a highly stable and accurate crystal oscillatoroperating at a relatively high frequency, for example 10 megahertz. Thereference input signal is applied to the servo22 through a summingjunction 29. Also connected to the summing junction 29 is a perturbationgenerator 31 that is synchronized to the reference input generator 28through a line 32. The output of the summing junction 29 is connected toa servo motor 33 which rotates an encoder 34 under test on a shaft 35.The speed of the servo motor 33 is controlled by the output of thesumming junction 29. The encoder 34 provides an output to the line 23,whichis negatively added to the summing junction 29, thereby completingthe feedback loop of the servo system 22. The lines 23 and 27 are alsoconnected to a correlator 37 that correlates the signals on the twolines and provides a test output 38.

Referring next to FIG. 2, there is shown the evaluator circuit 24 ingreater detail. The output line 23 is connected to a first angularposition sensor 41 and a sec- .ond angular position sensor 42. Theangular position sensor 41 produces an output only when the encoderunder test 34 is in a predetermined; specific angular position. Theangular position sensor 42 operates similarly except that response is toa different predetermined angular position', the responses of the twosensors 41, 42 thereby detecting an angular test segment with amagnitude of, for example, 2. A high precision crystal oscillator 43supplies on line 45 digital, timing pulses to a counter 44 therebyestablishing a measurement clock. The oscillator pulses are fed also onlines 50 into similar counters-in the evaluator circuits 25....26.Signals from the sensor 41 pass through a line 46 and start the counter44 and signals from the sensor 42 pass through line 47 and stop thecounter. The output of the counter 44 is carried bya line 48 toacomparator 49 that compares the output to a predetermined theoreticalvalue. Any difference detected by the comparator 49 is passed through abussbar S1 to AND gates 52, 53, 54 and 55. The sensors 41 and 42 eachrespond once during each revolution of the encoder under test 34, andthe outputs of the sensors are connected to a logic circuit 56 by thelines 46 and 47. The logic circuit 56 controls enabling inputs 52a, 53a,54a and 55a of the AND gates 52, 53, 54 and 55, and responds to theoutputs of sensors 41 and 42 so that the AND gates are-enabledsequentially. That is, the AND gate 52 is enabled to pass the differencedetected by the comparator 49 during the first revolution of the en-During operation of embodiment 21 the output of the perturbationgenerator 31 is normally setto zero and the reference input generator 28produces a highly stable reference input signal. The speed of the shaft35 is therefore held constant and regulated by the reference inputgenerator 28, subject only to minor torque variations and variations ofa compensatory nature from the feedback loop carried to the summingjunction 29 by the line 23. The complete output signal on the line 23represents the shaft angle of the shaft 35 and the encoder 34 asindicated by the calibrations-on the encoder, and is in the form ofbinary words that are used by evaluation circuits 24, 25....26, but onlythe least significant bits of which are passed through summing junction29, and that is in the form of periodic pulses. The reference input onthe line 27 is also in the form of periodic pulses and is of the samefrequency as the least significant bit output of the encoder. A phasecomparison of the two signals is'made in the summing junction 29 and ifthey are in phase the signal passed to motor 33 maintains the motor atits current speed;

' ations are compensated for. However, small errors in the calibrationofthe encoder 34that the system 21 is to detect will also cause a phaseshift in the output and therefore a phse discrepancy at the summingjunction 29. The servo system 22 should not follow these phasedifferences or the very errors sought to be detected will be masked.Calibration errors of the encoder 34 cause high frequency correctionsignals, but error correction signals due to improper speed of the motor33 are of a low frequency. The motor 33 and the shaft 35 are designedwith very high inertia and will change speed only in response to lowfrequency error signalsQtherefore the servo System22 will not respond tohigh frequency components in the output signal due to calibration error,but will correct for errors due to improper motor speed. The outputsignal on line 23 therefore includes all errors due to encodercalibration.

Referring now to FIG. 2'there is shown a block diagram of the evaluationcircuit 24 wherein the whole word position signals on the line 23 areapplied to the angular position sensors 41 and 42. Each sensor 41, 42 ispreselected to be responsive to a different word, or angular position ofthe encoder 34, and each sensor is activated only when the associatedencoder word is equal to the preselected position word. When the counter44 receives a pulse from the sensor 41 on the line 46 all informationstored in the counter is cleared and counting of the digital timingpulses from the crystal clock 43 begins. When the encoder 34 hastraversed the angular test segment and the counter 44 receives a pulseon the line 47 from the sensor 42 the counting stops, and the count,representing the time taken to traverse the angular tes t segment,appears on line 48. Since the angular velocity of the shaft 35 and henceof the encoder 34 is known and the counting frequency of the crystalclocks 43 is known and highly stable, the size of the angular testsegment can readily be determined from the count available on the line48. Al-

though the count available on the line 48 contains the error informationsought, it may be convenient to pro vide further data reductionautomatically, and this is done with the remaining optional componentsof the evaluation circuit 24. In the comparator 49 a comparison is madewith the expected theoretical value of the count and the output on line51 represents the difference between the actual count measured and thetheoretical count. The comparator 49 is optional and only reduces thecount from a large number (perhaps of the magnitude of 10 to adifference between actual and theoretical, which is several orders ofmagnitude lower and hence easier to work with. The common buss 51 takesthe difference from the comparator 49 to the AND gates 52, 53, 54 and55. The logic circuit 56 is responsive to the pulses on the lines 46 and47 and sequentially enables the AND gates 52, 53, 54 and 55 so that thedifference obtained from the comparator 49 during the dirst 360revolution of the encoder 34 is stored in the digital reigster 57 andthe difference obtained during the second revolution is stored in thedigital register 58 and so on. The information in the digital registers57, 58, 59 and 61 is fed into the averager 62 and averaged, and theaverage difference is available at the output 63. The error, in terms ofdegrees, is a function of the input reference frequency and the crystalclock frequency, both of which are known and accurate to one part in 10and the average difference in terms of clock count available at theoutput 63. The evaulation circuit 24 therefore provides automaticaveraging and integration of raw data.

Other options for data processing are available and deserve mention. Thefirst is either the elimination of the comparator 49 or the inclusionthereof at the output 63, in which case the information obtained at theoutput is the same. The second is the inclusion of only one digitalregister 57 that records the total number of counts from a plurality ofrevolutions. A division operation in the averager 62 then yields anaverage figure. Other components may be included to calculate standarddeviation if desired.

A test for improving the credibility of the system 21 will be reviewedwith reference to FIG. 1. Identical sig nals fed into the correlator 37would produce an output at the test output 38 of a constant level. Sincethe reference input signal and the output signal on the line 23 arepulse trains of the same frequency and closely in phase, a constantoutput will be available at the test output 38. lf the servo system 22were an ideal servo the output on the line 23 would always be identicalin frequency and phase angle with the input on the line 27, that is tosay if errors due to encoder calibration were detected by the summingjunction 29 and compensated for by the servo, the two input signals ofthe correlator 37 would be identical, and the output would be at'aconstant level. in practice, such is not the case. The minor errors dueto calibration of the encoder 34 are reflected in the output signal online 23 and appear at the test output 38 in the form of white noisesuperimposed on the constant level. The long term average of the whitenoise component is zero since the long term average encoder error iszero but over any short time sample, the white noise component ispresent at the test output 38 and represents encoder error notcompensated for by the servo system 22. The perturbation generator 31 isnext set to produce an output of a frequency that the servo system 22can follow. The perturbation signal must be synchronized to thereference input signal to assure that the long term average (an averagetaken over many revolutions of the encoder 34) of the perturbationsignal will not equal zero. A synchronization signal is delivered to theperturbation generator by line 32 from the reference input generator 28.The perturbation generator 31 produces a signal similar to that producedby a sustained phase difference caused by a change of speed, and if theservo system 22 will follow the perturbation produced by theperturbation generator then speed control correction signals will befollowed. An example of a typical perturbation signal is one thatincreases the speed of the motor 33 for three degrees of rotation of theencoder 34 and decreases the speed for the next three degrees andrepeats this cycle 60 times during each 360 revolution of the encoder.Since the servo system 22 does follow the perturbation signal, theperturbation is reflected in the output on line 23 which, whencorrelated with the reference input 27 produces the output describedabove, that is, a constant level with a white noise componentsuperimposed thereon; and in addition, analternating voltageproportional to the output of the perturbation generator, thus showingthat the servo system 22 does respond to error signals caused by speedvariations in the motor 33. An alternate method'of testing the responseof the servo system 22 to the signalfrom the perturbation generator is acomparison of the size of a plurality of angular test segments asmeasured normally, and as measured with the perturbation generator on.Test segments measured during a portion of the revolution of the encoder34 wherein the shaft speed is faster than the average speed because ofthe perturba tion generator 31 appears smaller than when measured withthe perturbation generator off. Similarly, segments through which theencoder moves more slowly appear larger. A more accurate measurementresults if a plurality of revolution of the encoder are used with theperturbation generator 31 on. Since the perturbation generator 31 issynchronized to the reference input generator 28 the phase relationshipremains constant during successive revolutions.

Referring now to FIG. 3 there is shown an alternate evaluation circuit24a for a second preferred embodiment 21a. In this embodiment 210 theservo system is the same as that represented by number 22 and depictedin FIG. 1, and the same information is available on the output line 23.The embodiment 21a, similar to the embodiment 21, may comprise one ormore evaluation circuits 24a. The output signal on'the line 23 isconnected to both a position sensor 65 and an angular position recoder66. The sensor 65 produces an output pulse only when one specific wordrepresenting a predetermined angular position is detected by thetransducer 36, and then a pulse is produced and passed on a line 67 tothe recorder 66, a crystal interval timer 68 and a logic circuit 69. Anoutput from recorder 66 is passed by a line 71 to a comparator 72 and anoutput thereof is passed by a bussbar 73 to AND gates 74, 75, 7-6 and77. Enabling inputs of the AND gates 74, 75, 76 and 77 are connected tothe logic circuit 69, and the outputs of the gates are connected to fourdigital registers 78, 79, 81 and 82. An integrator 84 is responsive tothe information stored in the digital registers 78, 79, 81 and 82 andproduces an output at an output terminal 83.

During operation of the embodiment 21a the servo system 22 operates asdescribed previously with respect to the embodiment 21. When the wordassociated with the sensor appears on the line 23, an output pulseappears on the line 67 that clears the position recorder 66 ofpreviously recorded information, startsthe crystal interval timer 68 andcauses the logic circuit 69 to enable theproper AND gate 74, 75, 76 or77. Upon the expiration of the timing period the timer 68 enables therecorder 66, and the angular position of the encoder 34 as indicated bythe calibrations thereon is recorded and appears on the line 71. Sincethe shaft speed and timing interval are accurately known, a theoreticalangular position may be easily calculated and compared withtheinformation available on the line 71. However, for averaging a pluralityof runs and simplifying data reduction the following optional circuitrymay be included.

- The measured position is compared within the comparator 72 to thetheoretical position and the difference therebetween appears on thebussbar 73. As noted above, each time the recorder 66 is cleared thelogic circuit 69 enables the following sequential AND gate such that thedifference obtained during the first 360 revolution of the encoder 34 ispassed to the digital register 78, and that difference obtained duringthe second revolution is passed to the digital register 79 and so on. Asdescribed previously with respect to evaluation circuit 24, a finalaverage value is calculated in the inte grator 84 and available atthe'output terminal 83. As with the evaluator circuit 24, the comparatormay be moved to a different position within the circuit, or may beeliminated entirely, similarly many revolutions may be averaged in asingle digital register 78;

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is to be understood,therefore, that the invention can be practiced otherwise than asspecifically velocity andincluding a transducer means operativelyassociated with'the encoder under test for producing an output signal;c. a first sensing means operatively associated with the encoder undertest for detecting a first prede termined angular position thereof;

d. a second sensing means operatively associated with the encoderundertest for detecting a second predetermined angular position thereof;

e. a clock means operatively associated with said first sensing meansand said second sensing means for measuring the time elapsed as theencoder rotates from said first predetermined angular position to saidsecond predetermined angular position; and

I means for taking a plurality of readings of the time elapsed as theencoder rotates from said first predetermined angular position to saidsecond predetermined angular position during a plurality of revolutionsof the encoder to average out errors in the system unrelated to errorsin the encoder.

2. An encoder evaluation system according to claim 1 including acorrelator means for correlating the reference input signal and theoutput signal of said transducer means.

3. An encoder error evaluation system according to claim 1 wherein saidservo means comprises a perturbation generator means for introducing apredetermined perturbation in the reference input signal.

4. A system as set forth in claim 1 wherein said means for taking aplurality of readings includes means for storing a plurality of valueswith each of said plurality of values representing the elapsed time forthe encoder to rotate from said first predetermined angular position tosaid second predetermined angular position during a particularrevolution of the encoder, whereby errors in the system unrelated toerrors in the encoder are averaged out .by the plurality of storedvalues.

5. An encoder evaluation system according to claim 4 wherein saidstoring means comprises a digital register means.

6. An encoder error evaluation system according to claim 5 wherein saiddigital register means comprises a plurality of digital registers, eachresponsive to the time measured by said clock means during a differentone of the angular motions.

7. An encoder error evaluation system according to claim 1 wherein saidmeans for taking a plurality of readings includes a comparator means fordetecting differences between the times measured during differentrotations of the encoder and a predetermined theoretical time.

8. An encoder error evaluation system according to claim 1 wherein saidclock means produces a digital timing signal.

9. An encoder error evaluation system according to claim 8 wherein saidclock means comprises a crystal oscillator.

10. An encoder error evaluation system according to claim 1 wherein saidreference input generator means comprises a crystal oscillator. I

11. An encoder error evaluation system according to claim 1 wherein:

a. said first sensor means comprises a plurality of first sensors, eachof said first sensors adapted to determine a distinct first angularposition of the encoder under test;

b. said second sensor means comprises a plurality of second sensors eachof said second sensors adapted to determine a distinct second angularposition of the encoder under test, and each of said second sensorsbeing associated with a different one of said first sensors so as toestablish therewith a discrete angular test segment; and

c. said clock means comprises a plurality of separate and independentclocks, each of said clocks being operatively associated with adifferent one of said first sensors and adapted to measure the timeelapsed as the encoder under test rotates through a different one of thediscrete test segments.

12. An encoder error evaluation system according to claim 11 including acomparator means comprising a plurality of separate and independentcomparator sections, each one associated with a different one of saidplurality of clocks, each of said comparator sections adapted to detectdifferences between the time measured by a different one of saidplurality of clocks and a predetermined theoretical time.

13. An encoder error evaluation system comprising:

a. a reference input generator means for producing a reference signal;

b. a phase-lock servo means responsive to the reference signal, saidphase-lock servo means adapted to rotate an encoder under test at aknown angular velocity and including a transducer means operativelyassociated with the encoder under test for producing an output signal;

c. an angular sensing means operatively associated with the encoderunder test for detecting a predetermined angular position thereof;

d. a timing means operatively associated with said angular sensing meansfor measuring a predetermined time interval beginning when said angularsensing means detects the predetermined angular position;

e. an angular position recorder operatively associated with said timingmeans for determining the angular position of the encoder under test asindicated by the output signal at the expiration of the predeterminedtime interval; and

f. means for taking a plurality of readings of the angular positiondetermined by said angular position recorder during a plurality ofrevolutions of the encoder to average out errors in the system unrelatedto errors in the encoder.

14. An encoder error evaluation system according to claim 13 including acorrelator means for correlating the reference input signal and theoutput signal of said transducer means.

15. An encoder error evaluation system according to claim 13 whereinsaid servo means comprises a perturbation generator means forintroducing a predetermined perturbation in the reference input signal.

16. A system-as set forth in claim 13 wherein said means for taking aplurality of readings includes means for storing a plurality of valueswith each of said plurality of values representing an angular positiondetermined by said angular position recorder during a particularrevolution of the encoder, whereby errors in the system unrelated toerrors in the encoder are averaged out by the plurality of stpredvalues.

17. An encoder error evaluation system according to claim 13 whereinsaid storing means comprises a digital register means.

18. An encoder error evaluation system according to claim 17 whereinsaid digital register means comprises a plurality of digital registorseach responsive to the indicated positions-determined by said angularposition recorder during a different one of the sequential rotations ofthe encoder under test. Y

19. An encoder error evaluation system according to claim 13 whereinsaid means for taking a plurality' of readings includes a comparatormeans for detecting differences between the angular positions determinedduring different revolutions of the encoder and a predeterminedtheoretical angular position.

20. An encoder error evaluation system according to claim l3 whereinsaid timer means produces a digital timing signal.

21. An encoder error evaluation system according to claim 20 whereinsaid timer means comprises a crystal oscillator.

22. An encoder error evaluation system according to claim 21 whereinsaid reference input generator means comprises a crystal oscillator.

1. A system for testing for errors in an angular encoder which convertsan angular position into a representative digital signal and comprising:a. a reference input generator means for producing a reference signal;b. a phase-lock servo means responsive to the reference signal, saidphase-lock servo means adapted to rotate an encoder under test at aknown angular velocity and including a transdUcer means operativelyassociated with the encoder under test for producing an output signal;c. a first sensing means operatively associated with the encoder undertest for detecting a first predetermined angular position thereof; d. asecond sensing means operatively associated with the encoder under testfor detecting a second predetermined angular position thereof; e. aclock means operatively associated with said first sensing means andsaid second sensing means for measuring the time elapsed as the encoderrotates from said first predetermined angular position to said secondpredetermined angular position; and f. means for taking a plurality ofreadings of the time elapsed as the encoder rotates from said firstpredetermined angular position to said second predetermined angularposition during a plurality of revolutions of the encoder to average outerrors in the system unrelated to errors in the encoder.
 2. An encoderevaluation system according to claim 1 including a correlator means forcorrelating the reference input signal and the output signal of saidtransducer means.
 3. An encoder error evaluation system according toclaim 1 wherein said servo means comprises a perturbation generatormeans for introducing a predetermined perturbation in the referenceinput signal.
 4. A system as set forth in claim 1 wherein said means fortaking a plurality of readings includes means for storing a plurality ofvalues with each of said plurality of values representing the elapsedtime for the encoder to rotate from said first predetermined angularposition to said second predetermined angular position during aparticular revolution of the encoder, whereby errors in the systemunrelated to errors in the encoder are averaged out by the plurality ofstored values.
 5. An encoder evaluation system according to claim 4wherein said storing means comprises a digital register means.
 6. Anencoder error evaluation system according to claim 5 wherein saiddigital register means comprises a plurality of digital registers, eachresponsive to the time measured by said clock means during a differentone of the angular motions.
 7. An encoder error evaluation systemaccording to claim 1 wherein said means for taking a plurality ofreadings includes a comparator means for detecting differences betweenthe times measured during different rotations of the encoder and apredetermined theoretical time.
 8. An encoder error evaluation systemaccording to claim 1 wherein said clock means produces a digital timingsignal.
 9. An encoder error evaluation system according to claim 8wherein said clock means comprises a crystal oscillator.
 10. An encodererror evaluation system according to claim 1 wherein said referenceinput generator means comprises a crystal oscillator.
 11. An encodererror evaluation system according to claim 1 wherein: a. said firstsensor means comprises a plurality of first sensors, each of said firstsensors adapted to determine a distinct first angular position of theencoder under test; b. said second sensor means comprises a plurality ofsecond sensors each of said second sensors adapted to determine adistinct second angular position of the encoder under test, and each ofsaid second sensors being associated with a different one of said firstsensors so as to establish therewith a discrete angular test segment;and c. said clock means comprises a plurality of separate andindependent clocks, each of said clocks being operatively associatedwith a different one of said first sensors and adapted to measure thetime elapsed as the encoder under test rotates through a different oneof the discrete test segments.
 12. An encoder error evaluation systemaccording to claim 11 including a comparator means comprising aplurality of separate and independent comparator sections, each oneassociated with a different one of said plurality of clocks, each ofsaid comparator sections adapted to detect differences betweeN the timemeasured by a different one of said plurality of clocks and apredetermined theoretical time.
 13. An encoder error evaluation systemcomprising: a. a reference input generator means for producing areference signal; b. a phase-lock servo means responsive to thereference signal, said phase-lock servo means adapted to rotate anencoder under test at a known angular velocity and including atransducer means operatively associated with the encoder under test forproducing an output signal; c. an angular sensing means operativelyassociated with the encoder under test for detecting a predeterminedangular position thereof; d. a timing means operatively associated withsaid angular sensing means for measuring a predetermined time intervalbeginning when said angular sensing means detects the predeterminedangular position; e. an angular position recorder operatively associatedwith said timing means for determining the angular position of theencoder under test as indicated by the output signal at the expirationof the predetermined time interval; and f. means for taking a pluralityof readings of the angular position determined by said angular positionrecorder during a plurality of revolutions of the encoder to average outerrors in the system unrelated to errors in the encoder.
 14. An encodererror evaluation system according to claim 13 including a correlatormeans for correlating the reference input signal and the output signalof said transducer means.
 15. An encoder error evaluation systemaccording to claim 13 wherein said servo means comprises a perturbationgenerator means for introducing a predetermined perturbation in thereference input signal.
 16. A system as set forth in claim 13 whereinsaid means for taking a plurality of readings includes means for storinga plurality of values with each of said plurality of values representingan angular position determined by said angular position recorder duringa particular revolution of the encoder, whereby errors in the systemunrelated to errors in the encoder are averaged out by the plurality ofstored values.
 17. An encoder error evaluation system according to claim13 wherein said storing means comprises a digital register means.
 18. Anencoder error evaluation system according to claim 17 wherein saiddigital register means comprises a plurality of digital registors eachresponsive to the indicated positions determined by said angularposition recorder during a different one of the sequential rotations ofthe encoder under test.
 19. An encoder error evaluation system accordingto claim 13 wherein said means for taking a plurality of readingsincludes a comparator means for detecting differences between theangular positions determined during different revolutions of the encoderand a predetermined theoretical angular position.
 20. An encoder errorevaluation system according to claim 13 wherein said timer meansproduces a digital timing signal.
 21. An encoder error evaluation systemaccording to claim 20 wherein said timer means comprises a crystaloscillator.
 22. An encoder error evaluation system according to claim 21wherein said reference input generator means comprises a crystaloscillator.